1. Field of the Invention
The present invention relates to a solar concentrator, particularly a space solar concentrator, to form a solar panel with rows of collapsible reflectors alternating with rows of solar cells.
2. Description of Prior Art
Spacecraft typically carries solar cells as a primary energy source. The solar cells are positioned and oriented on the spacecraft so that they are exposed to solar radiation.
On body-stabilized spacecraft, solar cells are typically arranged in planar arrays and carried on solar wings, which extend from opposite sides of a spacecraft body. Preferably, the solar wings rotate to keep them as orthogonal to the solar radiation as possible. Because the solar wings can be quite long in their deployed configuration, they are generally formed of a plurality of planar solar panels which are coupled together in an accordion arrangement (one-dimensional deployment) or in a paving arrangement (two-dimensional deployment) so that they can be collapsed to a smaller stowed configuration for spacecraft launch.
The number of solar cells that must be carried by a spacecraft is a function of the anticipated spacecraft power demand and the efficiency of the solar cells. Although high-efficiency solar cells reduce the number of cells required by a specific spacecraft, they are quite expensive. Because weight and weight-related costs also increase with the number of solar cells, there is a considerable incentive to reduce the quantity of solar cells that a spacecraft must carry.
Accordingly, efforts have been extended to concentrate solar radiation upon solar cells by using reflective surfaces that are positioned adjacent to solar panels and oriented to reflect additional radiation onto the cells. Solar radiation that would otherwise have passed by a solar wing is thus redirected to be incident upon the solar cells. Although a solar cell's efficiency in conversion of this additional reflected radiation to useful energy is typically less than it is for the directly incident radiation, primarily due to increased cell temperature and slanted angle of incidence, solar concentration allows the number of spacecraft solar cells to be significantly reduced with consequent savings in spacecraft weight and cost. Both rigid and flexible reflectors have been proposed for solar radiation concentration with flexible reflectors generally having a weight advantage. An exemplary flexible reflector system is shown in U.S. Pat. Nos. 6,017,002 and 6,050,526. An exemplary rigid reflector system is shown in U.S. Pat. No. 5,520,747.
Although these reflector systems concentrate solar radiation, their positioning adjacent to solar panel give rise to several drawbacks. The solar cell temperature increases and consequently the power conversion efficiency decreases. The pointing errors induce lack of flux uniformity on the cell panel and the power management is complicated, consequently decreasing the panel electric power collection.
In the case of deployable reflectors, the position of the reflectors and their deployment is not easily compatible with a two-dimensional deployment of the solar panels (paving-type panels) but only with a one-dimensional deployment thereof (accordion panels).
Another type of concentration with reflectors consists in distributing small reflectors on the solar panel. Reflectors are lying in between solar cell rows, alternatively. It reduces or cancels several of the mentioned drawbacks. The present invention is related to this kind of configuration. U.S. Pat. Nos. 6,188,012, 6,177,627, 6,528,716 and WO 00/79593 A1 are also describing some embodiments based on this geometric concept.
U.S. Pat. No. 6,188,012 applies only to a deployable concentrator. The deployment of the reflector is ensured thanks to several kinds of springs. After deployment, the spring is used to keep the reflective film under tension. The main drawback of such a device is the mechanical fatigue that occurs after a long time in space (with thermal cycling during each eclipse). For telecommunication spacecraft, the solar array must stay fully operational for 15 years in geostationary orbit. One eclipse per day occurs. A few thousand thermal cycles will result from a few thousand daily eclipses. If the reflector tension is progressively altered due to spring relaxation, the optical quality and the illumination uniformity will degrade. The effective concentration factor will vanish, with a significant loss for the spacecraft power generation. For that reason, after deployment, the reflective films need a fixer to ensure that no more mobility can produce the loss of tension. This patent is furthermore presenting deployment/storage concepts that are not fully valid. When the reflectors are stowed, their length looks smaller than when they are in deployed configuration. A realistic drawing would certainly depict that, in the stowed configuration, the reflector film is partially shading the solar cells. In case of reflector deployment failure, the reflective films are shadowing the solar cells and the resulting power generation is vanishing. This is another drawback that one aspect of the present invention intends to avoid.
U.S. Pat. No. 6,177,627 provides a compressible reflector apparatus comprising a flexible sheet and an erector under the sheet and mounted to a base. The sheet functions as a reflector for reflect lights. The erector supports the sheet. Thus two distinct parts are required.
WO 00/79593 A1 is presenting a concept with self-deployable reflectors. They are clearly shadowing the cells in the stowed configuration. There is no blocking mechanism after deployment. During storage, the solar panels are conventionally mounted in stack with small space in between. The stowed reflectors are using this available space but, since no locking mechanisms are present in the stowed configuration, the reflectors of panel i are collapsed against reflectors from the next panel (i+1).
This configuration is doubtful since vibration (during transportation and launch, for instance) could generate scratches on the reflective films, altering the optical quality and later the effective solar concentration with a loss of power generation.
U.S. Pat. No. 6,528,716 provides deployable solar concentrator having foldable reflectors. Similar to U.S. Pat. Nos. 6,188,012 and 6,177,627, the reflector needs both the erecting element and reflecting element for its proper operation. The compressible reflector also includes at least one reflector surface that must be folded when the concentrator is in stowed position.
In addition, all above references teach concentrators requiring a comparatively large amount of deployment energy.